The significance of the damping of vibrations generated in a motor vehicle and of external vibrations that are transmitted, for example, from the pavement to the vehicle, increases with increasing demands on comfort. This is of significance especially because of the necessary reduction of the noise level in the interior space and the reduction of vibrations that are perceived as being unpleasant.
Elastomer bearings are increasingly used, for example, to mount the drive assembly or movable components in the motor vehicle because of the favorable springing and damping properties of elastomer materials. The quality and the damping properties are substantially affected by the specific composition of the elastomer. The hardness and the elasticity of the bearings can be decisively affected by changing the material composition of the elastomer. However, a limit is set to this variability where great vibration amplitudes must be damped. These are generated, for example, when the drive assembly is idle or when motions act on the chassis periodically and in a shock-like manner on an uneven pavement. These so-called resonant vibrations can only be damped with conventional elastomer bearings or rubber bearings to a limited extent. However, since precisely resonant vibrations are felt to be very disturbing and very unpleasant in the vehicle, and moreover, these vibrations may bring about damage to expensive components, hydraulically damping elastomer bearings are increasingly used in modern motor vehicles.
These have at least two chambers, which are separated from one another, and in which a damping fluid is contained. The chambers are connected to one another via a flow channel and are deformed in case of an external force acting on the elastomer bearing, so that damping fluid can pass over from one chamber into the other. The chamber walls offer a resistance to the change in shape, which leads to a change in the pressure in the chambers. An indicator of this change in pressure due to the change in volume thus generated is called the “buckling spring rate.” To compensate the pressure difference between the chambers, the chambers are connected to one another via the flow channel already mentioned. Pressure equalization takes place between the chambers exclusively via this flow channel in case of spring compression with low frequencies. It follows from this that the elastomer bearing in this case makes a decisive contribution to the springing and damping of the elastomer bearing.
However, a damped system, which is capable of vibrating, and which comprises the elastic chambers walls and the mass of the damping fluid contained in the flow channel, is of increasing significance as the frequency increases.
If an elastomer bearing with hydraulic damping is excited in the range of a resonant frequency, the damping changes and the elastic properties of the elastomer bearing as a whole will change as well. Finally, the inertia of the amount of fluid present in the flow channel and the friction prevent a further pressure equalization between the chambers above the resonant frequency. As a result, the rigidity of the chamber walls supports the rigidity of the carrier and brings about an increase in the overall rigidity compared to the low-frequency type of load.
A hydraulically damping elastomer bearing, which is designed as an aggregate bearing for mounting the drive assembly of a motor vehicle according to the disclosure content of the document, is known from DE 101 04 458 A1. This has a housing as well as a connection element. The housing and the connection element are coupled with one another via an elastomer bearing in a vibration-damping manner. The housing and the connection element can thus be fastened at different components of the motor vehicle and consequently moved relative to one another. Furthermore, the hydraulically damping elastomer bearing known from DE 101 04 458 A1 has two chambers, which are filled with a fluid and which are connected to one another by a flow channel. This connection is designed as an open damping channel and makes possible the amortization of vibrations of the vehicle acting on the elastomer bearing, which amortization was already mentioned in the introduction. Furthermore, the prior-art elastomer bearing with hydraulic damping has an additional flow channel. This flow channel is designed as a bypass channel to the damping channel and is closed during the normal operation of the elastomer bearing. The bypass channel is opened only when needed, so that additional frequency ranges can hereby be damped, which would otherwise be passed on directly into the passenger compartment or to the motor vehicle. A valve, which acts on the bypass channel from the outside through a recess present in the housing and thus opens and closes same, is used to close the bypass channel. An actuator, which is in turn used to actuate the valve, is likewise located on the outside of the housing.
Also known are solutions in which the actuator is arranged directly within the elastomer bearing. JP 2002061702 A or JP 01135939 A are mentioned here as examples only. However, this is disadvantageous for several reasons. Thus, the properties of the damping fluid can be affected by the actuator, which is designed as an electromagnet in the prior-art solutions. In addition, such an elastomer bearing can be manufactured in a comparatively complicated manner, and the control lines for the actuator must be led off to the outside, which entails considerable sealing problems.
Compared to the embodiments with an actuator accommodated in the damping fluid, a hydraulically damping elastomer bearing, as is known from DE 101 04 458 A1, has the decisive advantage that neither the damping fluid within the bearing nor the actuator arranged therein is subject to mutual wear. The actuator arranged outside the elastomer bearing can be easily replaced when needed. No supply lines need to be led out of the elastomer bearing, and the elastomer bearing can be mounted in a simple manner. The actuator is an electromagnet, whose armature is formed by the valve. The valve itself reaches directly into the flow channel of the elastomer bearing via a piston made of an elastomer plastic. Thus, problems arise concerning the compliance with necessary tolerances during the manufacture of such an elastomer bearing, which makes mounting, on the whole, complicated and leads to an increase in the cost of the elastomer bearing in question. Another problem can be seen in the fact that the elastomer plug used in the prior-art solution is inserted into the flow channel designed as a bypass channel on the front side of the valve at right angles to the direction of flow and is thus exposed to high pressures at the vibration frequencies acting on the elastomer bearing. Consequently, this elastomer plug must have high stability, because its elasticity could lead to an impairment of the hydraulic bearing effect in case of a closed flow channel. Moreover, the sealing of the recess in the housing to avoid the escape of damping fluid from the elastomer bearing is relatively complicated.
Furthermore, a valve in the form of a diaphragm, which consists, on the whole, of an elastomer material, is known from JP 60-220239 A. Based on frequent switching operations during the opening and closing of the flow channel, this diaphragm is subject to extreme wear, so that the service life of an elastomer bearing of such a design is limited. Moreover, the closing force, which is necessary for closing the existing bypass channel, and which must be generated by the valve, is considerable, because the valve must be moved against the direction of flow of the fluid passing through the bypass channel in order to make it possible to close the bypass channel.